Browsing tag: antidote

We routinely sedate horses in practice – after vaccination, it’s probably the most common “routine” job that we do. So, what are we doing? How do the drugs work – and why doesn’t it always happen the same way?

“Sedation – a state of rest or sleep… produced by a sedative drug.”

That’s the dictionary definition, and it makes it sound lovely and simple – give a drug, and the patient goes to sleep. Of course, in reality (as usual with anything equine!) life isn’t that easy…

For those who haven’t seen it before, a sedated horse doesn’t lie down, but their head gets lower and lower, and they may require something to lean on to help them balance. It’s also important to remember that a sedated horse CAN still kick – they’re just much less likely to do so! It often seems that the horse is still more or less aware of what’s going on around them, but they’re too sleepy to care about it. As a result, we’d almost invariably use pain relief and local anaesthetic as well if we’re carrying out a surgical procedure.

There are a wide range of situations in which we like to use sedation. Generally, it’s to make the horse more amenable when something nasty or scary is being done to them. Of course, this varies from horse to horse. There are quite a lot of horses out there that need a sedative before the farrier can trim their feet; and there are others that will allow you to suture up a wound without sedation or even local anaesthetic (not recommended, but occasionally necessary).

Probably the most common reasons we sedate horses for are…

1) Stitching up wounds, to stop the horse wriggling!

2) Tooth rasping, especially when using power rasps and dremels

3) Some surgical operations – for example, many vets prefer to castrate colts under standing sedation, rather than a general anaesthetic. This is because sedation is much safer than a general anaesthetic… On the other hand, the surgery is easier and safer (for the vet, as well as the horse) if the patient is completely “out”, so it comes down to the type of horse and the preference of the vet doing the op.

It’s important to remember that all sedatives temporarily alter the way the horse’s brain and body works, and have a serious impact on the heart and circulatory system. As a result, they’re all prescription-only medicines, and your vet will want to satisfy themselves that the patient doesn’t have any underlying heart problems etc before using them. Overdose of a sedative is rarely fatal in a healthy horse, but it can still be dangerous, especially if there is any underlying illness that makes them less good at maintaining their blood pressure. Its also vitally important to tell your vet the horse’s whole medical history if you’re asking them to give a sedative – there have been cases of horses who were being treated with a (very safe) antibiotic (TMPS); the owner forgot to tell a vet this, and the combination of sedative and this antibiotic has resulted in a heart attack (technically, a fatal arrhythmia).

There are three routes by which we normally give sedation:

1) By syringe or in feed.
This is the slowest, least powerful and least reliable way to sedate a horse, but it has two advantages – you don’t need a vet to come and do it, and you don’t need to get so close to the horse to give it.
The drug most commonly used is ACP, sold as Sedalin or Relaquin paste. Occasionally ACP tablets are used, although there are strict restrictions on when a vet is allowed to prescribe tablets instead of paste. There is a newer drug now available as a syringe, detomidine (sold as Domosedan gel), which is absorbed across the membranes in the mouth so shouldn’t usually be given with food, but does work faster and give better sedation than ACP.

2) By injection into the muscle.
Many injectable sedatives can be given into the muscle – this injection is more reliable than by mouth, but requires much higher doses than if given into the vein (in my experience, you need 4-5 times as much, and it takes about twice as long to work). It’s only usually needed if the horse is too wild or dangerous to get a vein, but it’s quite useful to “take the edge off”, and then I can top up with intravenous sedatives if needed. The other situation where I’ve occasionally used it is when a severely colicing horse has to take a long ride in a box to get to a surgical centre. In these cases, I have sometimes given the driver a preloaded syringe so that if he horse freaks out or goes crazy in transit, they can give it something to calm it down and relieve the pain until they arrive.

3) By intravenous injection.
Intravenous sedation is by far the best option if possible – it works fast (usually 5-10 minutes), you need lower doses, and you get much better sedation than by any other route. This is what I’ll be concentrating on below.

There are three “families” of drugs used to sedate horses:

Acepromazine (ACP).
This is a very “dirty” drug, in that it affects a wide range of body systems. It can only produce mild to moderate sedation on its own, and the effects are very variable between horses. It’s important to remember that once sedation has been achieved; increasing the dose WON’T result in deeper sedation, just more side effects. It also has no painkilling properties.
There are two side effects in particular that we as vets watch out for with ACP. Firstly, it can lead to significant drop in blood pressure, because it makes peripheral blood vessels dilate (this is why it’s sometimes used in laminitis). The second effect is much more interesting – ACP is a mild muscle relacant of some muscle types, so it can be useful in azoturia and choke. There’s one exception though (male readers of a senstive disposition, look away now…): ACP is a very powerful relaxant for the retractor penis muscle. This is the muscle that holds the penis in the sheath, and even low doses of ACP usually lead to male horses “dropping” the penis. This can be useful, but unfortunately in some horses (especially stallions, with a larger and heavier penis than most geldings); the paralysis of the penis can be quite prolonged, which can result in penile trauma. In extreme cases, this can be permanent or lead to gangrene, requiring amputation. Bottom line – if at all possible, avoid using ACP in stallions and entire colts!
ACP does, however, have a place in sedation – when mixed with other drugs, it often prolongs sedation and means that the doses of each part of the combination can be dropped, reducing the risk of side effects.
A quick note on ACP tablets – under the current Veterinary Medicines Cascade laws, it is illegal to use ACP tablets instead of paste in horses unless the vet has a clinical reason (unfortunately, price isn’t considered good enough) to think that they are more appropriate. As a result, if your vet refuses to give you the tablets, they’re not trying to rip you off – they’re just obeying the law.

Opiates
Although opiates on their own are only very weak sedatives in horses, when combined with other drugs they lead to much deeper and smoother sedation than any other drug on its own. The drug usually used is butorphanol, which is a synthetic opiate (it’s a mu/kappa agonist/antagonist related to buprenorphine, for anyone interested) that has a fairly good painkilling effect as well as potentiating sedation from other drugs. Fortunately, it also has very few side effects, although its worth bearing in mind that any other opiates (e.g. Pethidine or Fentanyl) that the horse is given up to about 8 hours later won’t work quite like they’re supposed to, as the butorphanol will partially block their activity.

Alpha-2 Drugs
These really are the mainstay of sedation in horses (and in dogs and cats, for that matter). Alpha-2 drugs act by tricking the body into thinking it’s produced too much adrenaline, so it stops releasing it, resulting in reliable deep sedation. They’re also pretty powerful painkillers.
There are three drugs that are commonly used, with slightly different properties. Detomidine and Romifidine are both fairly long acting drugs (30-40 minutes after i/v use), and when mixed with butorphanol are the standard sedative preparation for intravenous use, or on their own into the muscle. Detomidine is also available in a syringe for oral use.
The third drug is xylaxine; this is a bit different in that it gives milder sedation, and only lasts 20 minutes or so. It’s particularly useful for sedating horses for nerve blocks etc, where in half an hour they need to be completely recovered and able to trot up.

Before I sedate a horse, I always have a good listen to the horse’s heart, and check its pulse and colour to make sure its cardiovasclar system is healthy. I’ll then double check it’s not on any medication, and then give i/v sedation.
I like to use either detomidine or romifidine mixed with butorphanol for routine sedation – I personally prefer detomidine, but that’s probably just because it’s what I “grew up” as a vet using! For longer lasting procedures, or if I want muscle relaxation (especially for dentals where I want the tongue nice and floppy!), I add ACP into the mix.
Dosage is incredibly variable between horses and experience and judgement is more important than all the book learning available. As a rule of thumb, the bigger the horse, the less sedative per kilo of body weight it needs (so Shetlands often need as much as a light hunter). In addition, it depends on temperament – the more highly strung or excited, the more sedatives are needed. The other thing to remember is that apparently identical horses, in the same circumstances, may react very differently – the dose that will have Alf so deep his head’s on the floor will have Brutus untouched, while Charlie is in the “Goldilocks” zone where he’s just right. Of course, it also depends how deep the sedation you want – although personally, I’ve found that if you aim for “light sedation” to start with, you usually end up having to top the horse up halfway through.
Once the injection’s been given, it is VITAL to give the horse time for it to work in a quiet, dim, calm place. If the horse gets excited while you’re waiting for the sedative to kick in, it won’t work well. This is doubly true for oral sedatives, but it applies to injections as well.
During the procedure, its sometimes necessary to top up, which is fine – the great thing about the drugs we use is that they work fast enough i/v that you can monitor their effects more or less in real time. Recovery is usually rapid and uncomplicated, although it’s important not to let the horse eat anything until it’s completely woken up, or it may choke.
Very occasionally, I’ve had a horse that refused to wake up, or went too deep. After my first one, I took to carrying the antidote (Atipamezole, aka Antisedan or Sedistop) with me when I sedated sick or old horses. It’s very expensive, but it works within a minute or two to reverse the effect of alpha-2 drugs – and once they’re reversed, the horse wakes up incredibly fast!

In practice, sedating horses is as much an art as a science, and there’s rarely one “right answer” – it depends on the horse, the circumstances, and what you’re trying to achieve. The main purpose is to allow us to treat your horse effectively and humanely.

If you are worried about any problems with your horse or pony, please talk to your vet or try our Interactive Equine Symptom Guide to help decide what to do next.